1. Field of Invention
The present invention relates to novel systems and methods for saturating a gas with a vapor from a liquid chemical. The invention also relates to novel systems and methods for controlled delivery of a vaporized liquid chemical. The invention has particular applicability to the semiconductor manufacturing industry.
2. Description of the Related Art
In the semiconductor manufacturing industry, high purity gases are supplied to process tools for carrying out various semiconductor fabrication processes. Examples of such processes include diffusion, chemical vapor deposition (CVD), etching, sputtering and ion implantation. The use of volatile liquids as the reactant source for these processes is known. Such liquids include, for example, silane (SiH.sub.4) dichlorosilane (SiH.sub.2 Cl.sub.2), trichlorosilane (SiHCl.sub.3), ammonia (NH.sub.3), boron trichloride (BCl.sub.3), chlorine (Cl.sub.2), hydrogen chloride (HCl) hydrogen fluoride (HF) and chlorine trifluoride (ClF.sub.3).
To date, a number of methods have been used to provide a vaporized form of the volatile liquid chemical to the process equipment. These include methods of saturating a carrier gas with a vapor of the liquid chemical and of vaporizing the liquid chemical without a carrier gas. Known methods of saturating a carrier gas with a chemical vapor include, for example, direct injection of the liquid chemical directly into a carrier gas stream. It is also known to bubble a carrier gas through the liquid chemical thereby saturating the gas with the chemical vapor.
In carrier gas using methods, the concentration in the carrier gas of the chemical vapor is affected by several factors. For example, bubble size, in terms of bubble surface area, and time of exposure of the bubble to the liquid chemical affect the chemical vapor concentration in the carrier gas. A known device employed in contacting a liquid chemical with a carrier gas is a perforated tube extending into a container holding the liquid chemical. The carrier gas is bubbled into the liquid chemical through the perforations in the tube, which permit the gas to be distributed to a limited extent in the liquid. As the bubbling progresses and the liquid chemical vaporizes, the level of the liquid chemical in the container continuously falls in the absence of fresh liquid chemical being added thereto. This variation in liquid level during the bubbling process results in a reduction in the gas-liquid contact time, thereby altering the concentration of the chemical vapor in the carrier gas.
Another variable affecting the concentration of the chemical vapor in the carrier gas is the temperature of the liquid chemical. The vapor pressure of the volatile liquid chemical is a function of the temperature of the liquid chemical. Thus, at any given temperature the carrier gas becomes saturated with the chemical vapor at an equilibrium condition. The carrier gas and chemical vapor coexist at their equilibrium saturation conditions as long as the temperature remains constant. If, however, the temperature drops, a portion of the chemical vapor condenses from the vapor state, resulting in a change in the concentration of the chemical vapor in the carrier gas.
To control the temperature of the saturation device, it is conventional to use a chiller unit alone. In the conventional device, the chiller unit cools the system to a sub-ambient temperature to prevent condensation of the chemical vapor from the carrier gas as it is delivered to the process equipment. This, of course, assumes that en route to the process equipment, the gas is not subjected to a temperature lower than that at which the saturation occurred. It has been found, however, that a chiller unit alone does not provide complete temperature control.
During the saturation process, conversion of the liquid chemical to a vapor results in the removal of additional heat from the liquid. The net effect of this heat removal is that the temperature of the liquid chemical may drop below the control temperature of the coolant. Because the chiller unit only provides cooling duty, this additional heat removal cannot be compensated for by the cooling system alone. Thus, variations in the vapor pressure of the liquid chemical and concentration of the chemical vapor in the carrier gas can result.
Still another variable which can affect the chemical vapor concentration in the carrier gas is the pressure of the carrier gas. Known devices have made use of mechanical pressure regulators, which rely on springs and diaphragms to sense and control pressure. With such mechanical devices, however, there is an inherent delay in responding to system changes. This can result in pressure fluctuations which, in turn, produce variations in the chemical vapor concentration.
In carrier gas-free methods, changes in the vapor pressure of the liquid chemical result from variations in the liquid chemical temperature. Such variations bring about changes in the delivery pressure and flowrate of the product vapor to the semiconductor processing equipment, leading to process instabilities.
To meet the requirements of the semiconductor manufacturing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide novel systems for saturating a gas with a vapor from a liquid chemical. Through the invention, a gas which has a substantially constant chemical vapor concentration can be provided. This is achieved through control of the carrier gas pressure, the temperature of the liquid chemical and the liquid chemical level in the saturation vessel. The degree of control now possible with respect to the properties of the product gas has not been achieved with systems to date.
It is a further object of the invention to provide novel methods for saturating a gas with a vapor from a liquid chemical. The methods can be practices with the inventive systems.
It is a further object of the invention to provide novel systems for controlled delivery of a vaporized liquid chemical, which systems are carrier gas-free.
It is a further object of the invention to provide novel methods for controlled delivery of a vaporized liquid chemical, which methods are free of a carrier gas source.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art on a review of the specification, drawings and claims appended hereto.